In a setback for a nonconformist engineer, Caltrans and the Los Angeles Bureau of Engineering have rejected his design for a new type of seismic base isolation to protect t he 6th Street Bridge near Downtown from earthquakes.

"We're not going to go and take a monolithic cement bridge, cut it apart and put in these base isolators," said Jim Roberts, chief structures engineer for Caltrans. "It costs a lot and it doesn't make sense."

Besides, said Roberts, the friction-slider isolation devices, which engineer Marc Caspe advocates over the rubber springs usually used in base isolation, have not been tested to his satisfaction.

In base isolation systems, what amount to shock absorbers are installed in or close to the foundation to insulate the major part of a structure from shaking. While still a somewhat uncommon form of earthquake mitigation, it has been used recently in more than a dozen major California projects, including six state highway bridges.

J. P. Ellman, president of the Los Angeles Board of Public Works, said that, on the city's side, Clark Robins, a senior city structural engineer, rejected the Caspe design for several reasons.

"Clark felt it was not cost-effective," Ellman said. "Caspe wanted to put the base isolator halfway up the bridge support, which didn't look too structurally sound and would cost an awful lot more money than a conventional retrofit. And Caltrans did not have much confidence in Caspe's design."

Caspe challenged the negative assessments of his design, done under a $48,000 city contract.

Although Caspe did install his devices at Berth 136 in the Los Angeles Harbor last spring to protect a new wharf, he has generally been unsuccessful in getting them accepted in buildings and bridges, although he contends that they cost much less than other types of earthquake protection.

The cost of his system for the 6th Street Bridge, at 4,000 feet the longest bridge in the city, would be $3.5 million, instead of $18 million for a conventional reinforcement, Caspe said Friday.

At Los Angeles City Hall, where cost estimates for a quake retrofit designed with base isolation as the key element have soared to $240 million, forcing a suspension of work and a reconsideration of the project, Caspe said his system would cost less than $50 million.

However, Robins, the city engineer, said Caspe's cost estimates do not include expensive additional bracing that would be needed as a consequence result of his design.

Caspe said Caltrans--whose advice is usually influential with municipal engineers--has foiled him with a Catch-22.

"It won't accept this technology because it claims it's unproven," he said. "Yet it can't be proven until we are allowed to install it in such structures."